How to fully protect your Mac against Zombieload bug, and how badly the fix affects performance

According to Apple, patches only go so far in protecting Macs from the Zombieload (MDS) bug, and the company recommends that customers who believe they are at heightened risk of attack should disable hyper-threading. Problem is, this has an enormous effect on performance.

Want to protect your Mac against the Zombieload (MDS) bug? According to Apple, becoming fully protected is not as simple as installing a patch.

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Step one, however, is to install the latest updates, which are MacOS 10.14.5 for systems running Mojave, and Security Update 2019-003 High Sierra and Security Update 2019-003 Sierra for Macs running older operating systems.

However, if you believe that you are at heightened risk of attack, Apple recommends that you disable hyper-threading on the CPU. And this comes with a huge performance cost, with Apple claiming "as much as a 40 percent reduction in performance with tests that include multithreaded workloads and public benchmarks."

That's a huge performance hit, and one that is likely to not only be noticeable, but have a major impact on how the Mac runs.

If you believe that you are at elevated risk from the Zombieload attack and want to disable hyperthreading, here's how to go about doing it:

1.     Apply the patches listed above.

2.     Switch on or reboot the Mac while pressing and holding Command (⌘)-R (or, for older Macs, one of the other macOS Recovery key combinations) on your keyboard.

3.     From the Utilities menu in the menu bar, click Terminal.

4.     Type the following at the Terminal prompt and press RETURN:

nvram boot-args="cwae=2"

5.     Type the following at the Terminal prompt and press RETURN:

nvram SMTDisable=%01

6.     Restart the Mac

To roll back this change, carry out an NVRAM reset (note that if you have to carry out an NVRAM reset for any other reason then you will need to manually disable hyper-threading again).

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